In recent years, there have been developed linear compressors as a mechanism for compressing and supplying refrigerant gas in a refrigeration system. As shown in FIG. 26, for example, a linear compressor includes a cylindrical housing 101 having a bottom, a magnetic frame 102 of a low carbon steel formed at the upper end opening of housing 101, a cylinder 103 formed in the central portion of magnetic frame 102, a piston 105 fit within cylinder 103, capable of moving back and forth and defining a compression chamber 104 in the space of cylinder 103, and a linear motor 106 serving as a driving source to drive piston 105 to reciprocate.
Linear motor 106 has an annular permanent magnet 107 provided at an outer concentric position with cylinder 103 and fixed to housing 101. A magnetic circuit formed of magnet 107 and magnetic frame 102 produces a magnetic field B in a cylindrical gap 108 concentric with the center of cylinder 103. A cylindrical mobile body 109 having a bottom, formed of resin and integrally fixed to piston 105 is provided in gap 108 in the center, and a coil spring 110 for elastically supporting mobile body 109 and piston 105 and permitting them to reciprocate is fixed to housing 101.
An electromagnetic coil 111 is wound around the outer circumference of mobile body 109 at a position opposite to magnet 107, ac current at a prescribed frequency is passed through a lead (not shown) to drive coil 111 and mobile body 109 by the function of a magnetic field through gap 108 to force piston 105 to move back and forth within cylinder 103, and gas pressure is generated at a prescribed cycle in compression chamber 104.
Meanwhile, as shown in FIG. 27, there is known, as a representative refrigerating system, a closed type refrigerating system in which a linear compressor 121 (compressor), a condenser 122, an expansion valve 123 and an evaporator 124 are connected by a gas flow path pipe 125. Linear compressor 121 is used as a device to compress to a high pressure a refrigerant gas evaporated at evaporator 124 and taken in through gas flow path pipe 125, and let out, thus pressurized, to condenser 122 through gas flow path pipe 125.
Therefore, as shown in FIG. 26, compression chamber 104 is connected with gas flow path pipe 125 outside housing 101 through a valve mechanism 112 provided at the upper end portion of cylinder 103. Valve mechanism 112 includes an inlet valve 112a which permits only refrigerant gas from evaporator 124 to enter through gas flow path pipe 125, and an outlet valve 112b which permits only refrigerant gas to be let out to condenser 122 through gas flow path pipe 125. Inlet valve 112a allows gas to flow toward compression chamber 104 by the difference in pressure of refrigerant gas between gas flow path pipe 125 on the low pressure side and compression chamber 104.
Outlet valve 112b allows gas to flow toward gas flow path pipe 125 on the high pressure side by the difference in pressure of refrigerant gas between compression chamber 104 and gas flow path pipe 125 on the high pressure side. Note that inlet valve 112a and outlet valve 112b are both energized by a plate spring.
Thus, in the conventional device, refrigerant gas taken in from inlet valve 112a is compressed to a high pressure in compression chamber 104, and supplied to condenser 122 through outlet valve 112b.
In addition, in recent years, as disclosed by Japanese Patent Laying-Open No. 2-154950, for example, there has been proposed a technique of improving the efficiency by providing compression chambers on both sides in a housing and alternately operating two pistons by a single linear motor.
The linear compressors are divided into two kinds, in other words, those like a coil mobile linear compressor as disclosed by Japanese Patent Application No. 8-179492, and those like a magnet mobile type linear compressor as disclosed by Japanese Patent Application No. 8-108908. These two kinds of linear compressors both produce compressed gas in a compression chamber by driving a piston to move back and forth using a driving force obtained from a linear motor.
The above-described linear compressors are, however, encountered with various problems as follows.
First Problem
The conventional single piston type linear compressor is largely affected by non-linear force produced within a compression chamber associated with inputting-king/compression/exhaustion of a gas, and the thrust of the motor cannot be linearized, which makes it difficult to improve the efficiency.
Furthermore, the neutral point of the piston fluctuates with the fluctuation of load at the time of activation for example, and the stroke of the piston cannot be readily controlled.
Second Problem
In a conventional linear compressor 121, piston 105 is driven by linear motor 106 to move up and down within cylinder 103, and mobile body 109 also moves up and down, which causes gas present in the space in the magnetic circuit formed by magnetic frame 102, permanent magnet 107 and mobile body 109, and gas present in the space inside the mobile body on the back side of piston 105 surrounded by the inner surface portion of mobile body 109 to perform compression/expansion work as mobile body 109 moves up and down, which could lead to irreversible compression losses in linear compressor 121.
As a countermeasure, gap 108 may be sufficiently secured to provide a sufficient gap between magnetic frame 102 and mobile body 109 and between permanent magnet 107 and electromagnetic coil 111, but the thrust of linear motor 106 decreases in this case, which lowers the operation efficiency of linear compressor 121.
Third Problem
In linear compressor 121 as described above, piston 105 is driven by linear motor 106 to move up and down within, and slidably in contact with, cylinder 103, and a kind of slide bearing is formed between the piston and the cylinder.
In the conventional structure as described above, however, a force (radial force) in the direction vertical to the moving direction of the piston is generated because of the problem of processing precision and a distortion in the electromagnetic force of the electromagnetic coil, and if the radial force is large, the operation efficiency may be lowered because of frictional losses, the life of the device may be shortened because of abrasion at a gas seal portion provided at piston 105, and the refrigerant may be contaminated by dust created by abrasion.
Fourth Problem
The linear compressor disclosed by Japanese Patent Laying-Open No. 2-154950 employs a magnet mobile type linear motor driving method rather than the coil mobile type as described above and shown in FIG. 26, force by magnetic field in the direction vertical to the moving direction of the piston is applied to the piston, the piston portion is prone to abrasion and therefore the compressor is not suitable for such use.
Therefore, in a linear compressor to be used for a long period of time, the driving method of the linear motor may be changed to the coil mobile type, according to which force by the magnetic field of the linear motor acts only in the same direction as the mobile direction of the piston.
Furthermore, gas present in the back space of the piston performs compression/expansion work as the piston moves back and forth, which could lead to irreversible compression losses in linear compressor 121.
In addition, in the conventional linear compressor, the central position of the stroke of piston cannot be controlled at a prescribed position, and therefore highly efficient operation cannot be performed.
Fifth Problem
In the refrigerating system as described above, compressed gas obtained in the compression chamber of the linear compressor is supplied to condenser 122 from outlet valve 112b through gas flow path pipe 125, vibration noise in the pipe caused by the pulsation of the gas or valve operation noise are generated at the time of opening/closing outlet valve 112b, and therefore there should be provided an outlet muffler 131 for controlling noise in the pipe on the downstream side of outlet valve 112b.
The above-described 2-piston type linear compressor must be provided with two such outlet mufflers for noise control, and two outlet pipes must be coupled preceding condenser 122, which could increase the size of the entire device.
Sixth Problem
In the refrigerating system as described above, the piston is permitted to move back and forth in the cylinder, and a coil spring is often used as a member for elastically supporting the piston to the housing. In recent years, a plate-shaped piston spring has been proposed which is advantageous over a conventional coil spring in terms of durability and positional restriction in the mobile direction, and various attempts have been made for improvements thereof (T. Haruyama, et al.: Cryogenic Engineering 1992 fall lecture meeting B2-4, p166).
The plate shaped piston spring is generally called a "suspension spring", and has a disk shaped plate spring 920a having a plurality of spiral cut out portions 920b equidistantly provided toward the central portion as shown in FIG. 28.
Using plate-shaped suspension spring 920 as the piston spring, the stroke central position of the piston can be fixed by a simple device.
Plate-shaped suspension spring 920, however, cannot restrict the deviation of the axis of the piston in the vicinity of upper and lower supporting points of the piston where the spring is fully extended. As a result, the piston may locally abut against the cylinder for some reasons and abrasion may be caused at the piston portion.
Seventh Problem
Meanwhile, the magnet mobile type linear compressor, as disclosed by Japanese Patent Application No. 8-108908, may be advantageously formed into a compact shape, but since attracting force by magnetic force is used as the driving force of the linear motor to force the piston to move up and down, force in the direction vertical to the upward and downward movement of the piston is likely to be generated. The driving force is lost because of friction between the piston and the cylinder and friction at the bearing portion of the shaft supporting the piston, which lowers the efficiency. Therefore, an expensive gas bearing, or the like, should be used for the bearing portion of the shaft supporting the piston.
The coil mobile type linear compressor as disclosed by Japanese Patent Application No. 8-179492, on the other hand, employs Lorentz force as the driving force of the linear motor, and therefore the deviation of the axis is less likely, as compared to the magnet mobile type linear compressor. In order to obtain the same output as by the magnet mobile type linear compressor, however, the device is generally increased in size.
It is therefore a first object of the invention to provide a highly efficient linear compressor which permits the stroke of a piston to be readily controlled.
Then, a second object of the invention is to provide a linear compressor whose efficiency is improved by reducing a gap in a magnetic circuit during the reciprocating movement of a mobile body as much as possible and preventing an irreversible compression loss.
Then, a third object of the invention is to provide a linear compressor whose efficiency is improved and whose life is prolonged.
Then, a fourth object of the invention is to provide a linear compressor having compression chambers on both sides in a housing, and compressing and externally supplying gas by driving a coil mobile type linear motor, wherein an irreversible compression loss is prevented in the back space of the piston by a simple structure, and the stroke central position of the piston is fixed.
Then, a fifth object of the invention is to provide a linear compressor having compression chambers on both sides in a housing, and compressing and externally supplying gas by driving a coil mobile type linear motor, wherein the stroke central position of the piston is fixed by a simple structure, abrasion at the piston portion is prevented by restricting the deviation of the axis of the piston when the piston is driven to reciprocate, and the life of the device is prolonged.
A sixth object of the invention is to provide a linear compressor which permits prevention of loss in the driving force, caused by friction between a piston and a cylinder and friction at the bearing portion of a shaft supporting the piston and the size of the device to be reduced.